1. Field of the Invention
The present invention relates to electric power converters. More specifically, the present invention relates to power converters having regulated dual outputs.
2. Description of Related Art
There is constant need in the electronics industry for efficient DC-to-DC or AC-to-DC power converters. The current trend is to develop low output voltage (e.g., 3.3 V and less), high output current (e.g., greater than 20 A), high efficiency, and high power density converters.
It is well known to form a converter circuit having a dual output. In one such known dual output converter circuit, both outputs of the converter are regulated. A regulating switch is arranged in series with a forward diode so that the switch is conducting during most of the xe2x80x9con-timexe2x80x9d of the converter. A drawback of this arrangement is that the conduction of the switch during most of the on-time increases the power dissipation in the circuit, and thus reduces the efficiency of the converter circuit. Further, during transient conditions when the duty cycle temporarily goes to zero, the second output may sag so as to exhibit poor cross regulation.
In a second conventional dual output converter circuit, a first output is regulated and a second output is quasi-regulated. The second conventional converter has inductors coupled to a common core so that the size of the converter circuit is reduced compared to the first conventional converter discussed above. This converter provides good power density and efficiency, but suffers from poor regulation, particularly at low voltages and high currents of the quasi-regulated output. Additionally, the initial voltage of the quasi-regulated output is determined by the voltage at the first output and the turns ratio of the secondary windings of the transformer. This puts severe limitations on the achievable voltages at the second output, again, particularly at low voltages and high currents. Thus, the second conventional converter is limited to applications that require only relatively low output currents.
Accordingly, it would be very desirable to provide a converter having well-regulated dual outputs with improved power density, improved efficiency and cross regulation, along with fewer limitations on the achievable output voltages.
The present invention provides a converter having well-regulated dual outputs. The converter according to the present invention also has the advantages of improved power density, efficiency, and cross regulation, while providing fewer limitations on the achievable output voltages.
In a first embodiment, a converter having first and second output terminals includes a transformer having a primary winding, and first and second secondary windings. On a primary side of the converter, there is provided the primary winding of the transformer, and a first switch having an activation terminal, where the switch is coupled to the primary winding. A first controller has its input connected to the first output terminal of the converter and its output connected to the activation terminal of the switch. On a secondary side of the converter, there is provided first and second secondary sections. The first section includes the first secondary winding and a first inductor coupled between the first secondary winding and the first output terminal. The second section includes the second secondary winding and a second inductor coupled between the second secondary winding and the second output terminal. The first and second inductors can be either independent or magnetically coupled. The first and second sections on the secondary side of the converter each further include a forward rectifier (e.g., diode or MOSFET) connected in series between the respective secondary winding and inductor, and a freewheeling rectifier (e.g., diode or MOSFET) connected in parallel with the respective secondary winding. The second secondary section also includes a second switch adapted to apply a regulating voltage to the second inductor during a regulating period of the converter. The regulating voltage increases the volt-seconds applied to the second inductor, which improves the regulation of the voltage at the second output terminal.
If the two output voltages are close, the conduction in the second switch is short in comparison with the period of the switching frequency of the converter circuit and therefore the power dissipation in the second switch is low. When the second switch is conducting, which occurs in a regulation period of the converter, the freewheeling rectifier of the second section is not conducting, thereby further reducing power dissipation. As regulating (auxiliary) voltage is applied to the second inductor during a portion of the off-time, lower on-time voltage amplitude is required to achieve desired secondary voltage. That reduces voltage stress on the rectifiers and therefore allows a further reduction of power dissipation by using low on-resistance devices such as MOSFETs or Schottky diodes. By controlling the regulation period, the voltage at the second output terminal is well regulated. As energy to the second output is supplied partially from the auxiliary voltage source, it further improves the dynamic cross-regulation between the two outputs.
In a second embodiment, a converter having first and second output terminals includes a transformer having a primary winding and a secondary winding. On a primary side of the converter there is provided the primary winding of the transformer, and a first switch having an activation terminal, where the switch is coupled to the primary winding. Also included is a first controller having its input connected to the first output terminal of the converter and its output connected to the activation terminal of the first switch. On a secondary side of the converter, there is provided the secondary winding. First and second coupled or independent inductors are connected through rectifiers to the secondary winding and the first and second output terminals, respectively. The second secondary section also includes a second switch adapted to apply a regulating voltage to the second inductor during a regulating period of the converter. The regulating voltage increases the volt-seconds applied to the second inductor, which improves the regulation of the voltage at the second output terminal.
If the turns ratio of the secondary windings of the transformer is equal to one, then a transformer having a single secondary winding may be used. Advantages of having one secondary winding include simpler transformer construction and improved cross-regulation of the output voltages at the first and second output terminals. Also, because there is only one secondary winding, the first controller provides load regulation for both output voltages. Accordingly, a second controller regulates the output voltage at the second output terminal only to the required output voltage level, which leads to a shorter regulating interval Treg, and consequently even less power dissipation as compared to the first embodiment. As in the first embodiment, the second switch conducts only during a portion of the off-time period of the converter referred to as the regulation period. This causes a regulating voltage to be applied to the second inductor and increases the volt-seconds applied to the second inductor so that the voltage at the second output terminal is well-regulated.
A more complete understanding of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of preferred embodiments. Reference will be made to the appended sheets of drawings, which will first be described briefly.